Hydroxyl Group | Structure, Formula & Examples – Video & Lesson Transcript | Study.com

Video Transcript

What Is a Hydroxyl Group?

Many people enjoy a glass of wine on any given day, or use rubbing alcohol to clean a wound. And there probably isn’t a person on Earth who doesn’t consume some form of sugar every day. But it’s highly unlikely that these same people are wondering about the chemical makeup of alcohol and sugar. In this lesson, we will learn about a key component in the chemical makeup of sugars and alcohols. While these substances have many different chemical compositions, sugars and alcohols all have two things in common: they are all carbon-based, and they all contain a pair of atoms called the hydroxyl group.

Before we get into the specifics of hydroxyl groups, it is important to get a better understanding of carbon-based molecules. Organic chemistry is the study of carbon-based molecules. These molecules, otherwise known as organic molecules, are so important that life could not exist without them. DNA, proteins, and carbohydrates are examples of organic molecules that are necessary for every living being. And the hydroxyl group is one of their essential building blocks.

Chemical Formula and Structure of the Hydroxyl Group

A hydroxyl group is composed of one hydrogen atom bonded to one oxygen atom. Its chemical formula is written as either -OH or HO-. The ‘-‘ represents the carbon to which the hydroxyl group is bonded.

The R in the structural formula stands for the carbon backbone of the organic molecule to which the hydroxyl attaches.

Functional Groups in Organic Molecules

To further explore the specifics of a hydroxyl group, let’s go back to our basic understanding of an organic molecule. Organic molecules are carbon-based and also may contain oxygen, hydrogen, nitrogen, sulfur, and/or phosphorus. Structurally, these molecules are composed of two main parts.

The first part is the carbon backbone, in which the carbon atoms are bonded together forming a carbon backbone.

The second are the functional groups, which are small groups of atoms, such as hydrogen and oxygen, that are bonded to the carbon backbone. Functional groups are so named because they function as the chemically reactive area of the molecule.

The hydroxyl group (-OH) is one example of a functional group. When hydroxyl groups are the primary functional group bonded to carbon backbones, the resulting molecules are alcohols. Here we see the structural formula for the organic molecule ethanol (a type of alcohol) with the hydroxyl group on the far right.

Methanol, isopropyl alcohol, and propanol are additional examples of alcohols containing the hydroxyl group.

Carbohydrate molecules, or sugars, have hydroxyl groups, too. However, sugars also contain another important functional group, called the carbonyl group (-CO), that alcohols don’t have. This is what distinguishes sugars from alcohols. Looking at the structure of a sugar called glucose, you can see that there are hydroxyl groups on each side of both examples.

DNA, which is the molecule containing the genetic code for every living organism, also has the hydroxyl group as part of its makeup. The ‘D’ in DNA stands for deoxyribose, which is a sugar molecule. Therefore, the hydroxyl group is also a key component in this very important molecule.

Hydroxyl and Polarity

Hydroxyl groups are polar. When we hear the word polar, we may think of the poles of the earth and polar bears. But what does this mean when it comes to molecules? Polarity has to do with electric charge. When a molecule is polar, this means that one side has a partial positive charge and one side has a partial negative charge. You can think of the poles of the earth being on far opposite sides. Likewise, in a molecule, the slight charges are on opposite sides. In a hydroxyl group, the oxygen side is negative and the hydrogen side is positive. Why is this important? Because polarity causes attraction between molecules.

We know that opposites attract, so the positive side of a molecule will be attracted to the negative side of another molecule. Water is a polar molecule as well, so it will be attracted to molecules containing hydroxyl groups. This is where we get to see the chemical reactivity of the functional groups at work. When sugars and certain alcohols come into contact with water, the water molecules react with them to dissolve them. The solubility of alcohols does vary, but the simpler molecules, such as methanol, ethanol, and propanol, will completely dissolve in water.

Lesson Summary

Hydroxyl groups are a functional group found in sugars and alcohols. A hydroxyl group consists of one hydrogen and one oxygen atom and can be written as either -OH or HO-. Hydroxyl groups are polar, and the oxygen side is always negative, while the hydrogen side is always positive.

The hydroxyl group is an integral component of organic chemistry. It may only consist of a pair of atoms, but without it, many important substances, such as sugars and alcohols, would not exist.

Learning Outcomes

When you are done, you should be able to:

• Describe the structure of a hydroxyl group
• Recall what sugars and alcohols have in common and differentiate between the two
• Name some common substances that contain hydroxyl groups
• Define organic molecules and list their two main parts
• Explain what the polarity of a hydroxyl group is and why it is important